1. Field of the Invention
The present invention concerns a method to generate raw data sets from echo acquisitions of magnetic resonance signals, and a corresponding computer-readable data storage medium and a magnetic resonance system that implements such a method.
2. Description of the Prior Art
New fields of use in magnetic resonance tomography are offered by the acquisition of magnetic resonance data (shortened to MR data) with very short echo times TE (for example TE<500 μs. It is thereby possible to depict substances or tissue that cannot be shown by means of conventional sequences—for example a (T)SE ((Turbo Spin Echo) sequence or a GRE (Gradient Echo) sequence)—since their T2 time (the relaxation of the transverse magnetization of this substance or tissue) is markedly shorter than the echo time, and thus a signal or ignition from these substances or tissues has already decayed at the acquisition point in time. For example, with echo times in the range of the corresponding decay time, it is possible to show bones, teeth or ice in an MR image, although the T2 time of these items lies in a range from 30-80 μs.
One approach to enable short echo times is to scan k-space (i.e. make data entries into k-space) in points detected during free induction decay (FID). Such a method is also designated as single point imaging because essentially only one raw data point in k-space is detected per RF excitation. One example of such a method for single point imaging is the RASP method (“Rapid Signal Point (RASP) Imaging”, O. Heid, M. Deimling, SMR, 3rd Annual Meeting, Page 684, 1995). According to the RASP method, a raw data point in k-space, whose phase has been coded by gradients, is read out at a fixed point in time after the RF excitation relative to the echo time TE. The gradients are modified by means of the magnetic resonance system for each raw data point or measurement point, and k-space is thus scanned point by point.
To create an image that depicts only substances or tissue which have a very short T2 time (bones, for example), it is typical to implement the RASP method twice, for example, wherein the RASP method operates with such a short echo time TE in the first pass that the bones (for example) still supply a signal, and wherein the RASP method operates with a correspondingly longer echo time TE in the second pass so that the bones no longer deliver a signal. Each pass of the RASP method respectively produces an image, and the two images created in such a manner are subtracted from one another so that only tissues or substances that have a very short T2 time are still shown in the resulting difference image.